Internet Engineering Task Force (IETF) D. Thaler
Request for Comments: 5991 Microsoft
Updates: 4380 S. Krishnan
Category: Standards Track Ericsson
ISSN: 2070-1721 J. Hoagland
Symantec
September 2010
Teredo Security Updates
Abstract
The Teredo protocol defines a set of flags that are embedded in every
Teredo IPv6 address. This document specifies a set of security
updates that modify the use of this flags field, but are backward
compatible.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5991.
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Thaler, et al. Standards Track [Page 1]RFC 5991 Teredo Security Updates September 2010
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Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Specification ...................................................4
3.1. Random Address Flags .......................................4
3.2. Deprecation of Cone Bit ....................................6
4. Security Considerations .........................................7
5. Acknowledgments .................................................7
6. References ......................................................8
6.1. Normative References .......................................8
6.2. Informative References .....................................8
Appendix A. Implementation Status .................................9
Appendix B. Resistance to Address Prediction ......................9
1. Introduction
Teredo [RFC4380] defines a set of flags that are embedded in every
Teredo IPv6 address. This document specifies a set of security
updates that modify the use of this flags field, but are backwards
compatible. This document updates RFC 4380.
The Flags field in a Teredo IPv6 address has 13 unused bits out of a
total of 16 bits. To guard against address-scanning risks [RFC5157]
from malicious users, this update randomizes 12 of the 13 unused bits
when configuring the Teredo IPv6 address. Even if an attacker were
able to determine the external (mapped) IPv4 address and port
assigned by a NAT to the Teredo client, the attacker would still need
to attack a range of 4,096 IPv6 addresses to determine the actual
Teredo IPv6 address of the client.
The cone bit in a Teredo IPv6 address indicates whether a peer needs
to send Teredo control messages before communicating with a Teredo
IPv6 address. Unfortunately, it may also have some value in terms of
profiling to the extent that it reveals the security posture of the
network. If the cone bit is set, an attacker may decide it is